We plan to fine-map and identify a gene on Chromosome 21 that causes familial amyotrophic lateral sclerosis (FALS). Defining the precise location of this gene will allow us to isolate it and study its function. Our mapping strategy is fourfold: (1) We will generate highly polymorphic DNA markers (such as (CA)n repeat(s) around the putative locus of the FALS gene on Chromosome 21 from previously mapped cosmid and yeast artificial chromosomes markers. These markers will be tested by linkage analysis in our FALS families in order to obtain the optimum genetic information from them. Crossovers between the disease (FALS) locus and the markers will help define the precise location of the FALS gene. (2) Once flanking markers are defined we will construct a band- specific genomic library by microdissection of chromosome 21. (3) We will test candidate FALS genes in our FALS families that are obtained from (a) spinal cord cDNA library that has been screened against the microdissected chromosome 21q band-specific library and (b) from subtracted cDNA libraries (made from the anterior and posterior gray matter as motor neurons undergo the most profound pathological change and are therefore prime candidate for expression of the FALS gene. Thus, if the FALS gene is represented in our clones, it should map to chromosome 21 and an appropriate polymorphism derived from it should segregate with the disease trait in our FALS families. This may provide a direct approach to the cloning of the FALS gene on Chromosome 21. (4) We have identified a Downs patient with an amyotrophic lateral sclerosis-like syndrome. This patient is a mosaic for trisomy 21, (46XY, normal 21 and marker 21) and a deleted chromosome 21 (46XY, normal 21 and (del)21). We plan to determine the extent of deletion in the deleted chromosome 21 as this may define the limits of the region containing the FALS gene. Yeast artificial chromosomes and cosmids from this region can then be used to generate markers for testing in our families. this robust approach in mapping the chromosome 21 FALS gene will allow its isolation and lead to an understanding of the fundamental defect in the degeneration of motor neurons. This information will be especially useful in formulating a rational treatment of this and related disorders that are otherwise fatal.